The invention relates to a method for actuating an accumulator catalyst for nitrogen dioxide on an internal combustion engine for a vehicle, in particular a car.
In current automotive engineering spark ignition engines as internal combustion engines with direct gasoline injection instead of conventional manifold injection are preferred, since these internal combustion engines, compared to conventional spark ignition engines, have clearly more dynamics, are superior with respect to torque and output, and at the same time makes possible a reduction in fuel consumption by up to 15%. This makes possible so-called stratification in the partial load range in which an ignitable mixture is required only in the area of the spark plug, while the remaining combustion chamber is filled with air. Since conventional internal combustion engines, which work according to the manifold principle, at such a high air excess as prevails in direct gasoline injection can no longer be ignited, in this stratified mode the fuel mixture is concentrated around the spark plug which is positioned centrally in the combustion chamber, while in the edge areas of the combustion chamber there is pure air. In order to be able to center the fuel mixture around the spark plug which is positioned centrally in the combustion chamber, a concerted air flow in the combustion chamber is necessary, a so-called tumble flow. In the process an intensive, roller-shaped flow is formed in the combustion chamber and the fuel is injected only in the last third of the upward motion of the piston. By the combination of the special air flow and the dedicated geometry of the piston which has for example a pronounced fuel and flow depression, the especially finely atomized fuel is concentrated in a so-called “mixture ball” optimally around the spark plug and reliably ignites. The engine control provides for the respectively optimized adaptation of the injection parameters (injection time, fuel pressure).
These internal combustion engines can therefore be operated in lean operation for a correspondingly long time; this has a beneficial effect on fuel consumption overall, as has been described in the foregoing. This lean operation however entails the disadvantage of a much larger amount of nitrogen oxide in the exhaust gas so that the nitrogen oxides (NOx) can no longer be completely reduced in the lean exhaust gas of a 3-way catalyst. In order to keep the nitrogen oxide emissions within the scope of prescribed limits, for example of the Euro-IV limit value, nitrogen oxide storage catalysts are generally used in conjunction with these internal combustion engines. These nitrogen oxide storage catalysts are operated such that the large amounts of nitrogen oxides produced by the internal combustion engine are stored in them. With the increasing amount of stored nitrogen oxide a saturation state is reached in the nitrogen oxide storage catalyst so that the nitrogen oxide storage catalyst must be discharged. In the process, for a so-called discharge phase switching to substoichiometric, rich engine operation takes place briefly by means of the engine control or engine control device, in which the internal combustion engine is operated with a rich mixture which has an air deficit in order to achieve discharge of the nitrogen oxide. In this discharge process the stored nitrogen oxide is reduced by the hydrocarbons (HC) and carbon monoxides (C), which are present in large amounts under these rich operating conditions, to nitrogen (N2) which can then be released into the environment.
According to a generally known, generic process for operating a nitrogen oxide storage catalyst of an internal combustion engine of a motor vehicle, there is a first operating range as the lean operating range in which the internal combustion engine is operated with a lean mixture and in which the nitrogen oxides contained in the exhaust gas flow are stored in a nitrogen oxide storage catalyst, to discharge the nitrogen oxide storage catalyst at a predeterminable switching instant when a predeterminable switching condition is satisfied by means of the control device switching taking place from the lean operating range to the rich operating range.
Specifically, for this purpose the discharge instant is computed by the engine control device using modeled values filed in the engine operating map. The problem in these model assumptions is however that the actual conditions often do not correspond to the modeled values and deviate from them. This is especially problematical when the modeled nitrogen oxide raw emission values which are included in the computation of the discharge instant in the exhaust flow do not agree with the actual nitrogen oxide raw emission values, especially the case being problematical in which the actual nitrogen oxide raw emission values are higher than the modeled nitrogen oxide raw emission values. Thus, deterioration of the actual nitrogen oxide raw emission values which is not detected by the model leads to a much earlier nitrogen oxide breakout of the nitrogen oxide storage catalyst. In these nitrogen oxide breakouts the required exhaust gas limit values are generally not complied with. To avoid this, there is a margin for the uncertainties with respect to the deterioration of the actual nitrogen oxide raw emission values which is not detected by the model in practical operation, i.e., a type of “safety interval” is dictated with respect to the predetermined exhaust gas limit values. But this margin results in that often discharge takes place at a time at which in fact a discharge did not need to be undertaken, since the storage potential of the storage catalyst is not fully used, i.e., overall more discharges than actually necessary will be carried out; this in turn increases fuel consumption in an undesirable manner.
Processes for modeling the actual conditions in a nitrogen oxide storage catalyst are known among others from EP 0 867 604 A1 in which the nitrogen oxide storage capacity is determined as a function of the temperature of the storage catalyst.
A process for operating a nitrogen oxide storage catalyst with a correction factor is known from EP 0 997 626 A1.
A model for computing the charging of a nitrogen oxide storage catalyst with nitrogen oxides and sulfur oxides is known from DE 100 38 461 A1.
Furthermore WO 02/14659 A1 discloses a process and a model for modeling the discharge phase of a nitrogen oxide storage catalyst in which an oxygen reservoir is modeled by a first integrator for oxygen and a nitrogen oxide reservoir is modeled by a second integrator for nitrogen oxides and the first integrator and the second integrator are subjected proportionally to the reducing agent mass flow according to a division factor, the division factor being determined depending on the oxygen reservoir contents and the nitrogen oxide reservoir contents of the nitrogen oxide storage catalyst.